This invention relates to a tone synthesis method in which a musical tone signal is synthesized by a frequency modulation operation or an amplitude modulation operation and, more particularly, to a tone synthesis method capable of controlling a relatively large number of frequency components by a simple operation.
Chowning, U.S. Pat. No. 4,018,121 discloses a fundamental technique for synthesizing a tone signal having a desired harmonic composition by a frequency modulation operation in which the carrier and modulating frequencies are both in the audio frequency range. For synthesizing a tone of a satisfactory tone color having sufficient harmonic components by employing the technique disclosed in this United States Patent however, a frequency modulation (abbreviated as FM hereunder) operation using a simple monomial expression is insufficient and it requires an FM operation of a multiplex or polynomial expression. This inevitably necessitates a synthesizing circuit with a complicated and large-scale construction and, in a system which performs the synthesis operation for each term on a time shared basis, a control clock of a high rate must be employed with a resulting high manufacturing cost. A tone synthesis technique using this type of multiplex or polynomial FM operation is disclosed in U.S. Pat. No. 4,253,367.
It has also been conceived, as a technique for synthesizing a tone containing abundant harmonic components by a relatively simple operation, to prestore a waveshape containing abundant frequency components in a waveshape memory and use its output as a modulation wave or a carrier wave. Since a waveshape usable for such operation is fixed to the one waveshape which has been stored in a waveshape memory, there is limitation in a tone color which can be synthesized by this technique. The same problem exists not only in the tone synthesis technique of the FM operation type but also in that of an amplitude modulation operation (abbreviated as AM hereunder) type.
It is an object of the present invention to provide, a technique of synthesizing a tone signal by a predetermined modulation operation, a method capable of synthesizing a tone signal having abundant frequency components with a relatively simple construction.